1. Field of the Invention
This invention relates to methods and apparatus for use in bead coating a web with liquid composition.
2. Description relative to the Prior Art
U.S. Pat. No. 3,220,877, to Johnson and issued Nov. 30, 1965, describes a method of coating a web with liquid composition, for example, photographic composition, using a slide hopper. Compositions to be coated in layers on a web are forced out of slots onto slide surfaces of the hopper. The materials slide down the slide surfaces, stacking up on one another as they flow down. The stacked layers come to the hopper lip at the bottom of the slide surfaces. Opposite the lip the web to be coated is moving upwardly around a coating roll which serves to back up the web at the coating point and to accurately space the web from the hopper lip. The layers flow off the hopper lip and bridge the gap and flow onto the web. The layers, when in the gap, are said to form a bead, from which the term "bead coating" is derived.
The aforesaid patent specification further describes the provision of a pressure differential across the bead, with the lower pressure being on the underside of the bead. The pressure differential is greater during the start of a coating operation and is lowered for the remainder of the coating operation, when the hopper has been backed away from its very close start-up position. The pressure differential is created by providing a casing under the bead, which, together with a portion of the hopper and a portion of the web on the coating roll, bound a chamber. Air is withdrawn from the chamber by a vacuum pump. The magnitude of the pressure differential is controlled by a regulating valve.
U.S. Pat. No. 3,916,043, to Fowble and issued Oct. 28, 1975, describes increasing the pressure differential when a splice in the web passes through the gap between the hopper lip and the web on the coating roll. Again, a regulating valve is provided in the conduit between the chamber under the bead, and the vacuum source.
It has been found that when a regulating valve is used to regulate the magnitude of the pressure differential across the bead, the pressure differential maintained is not constant. Variations, in the form of surges, in the pressure differential create variations in the uniformity of the laydown of the compositions on the web. Such variations in uniformity of laydown are particular apparent and objectionable in motion picture film which is used to present successive images, on successive portions of the film, to the eye. Furthermore, poor repeatability of the pressure differentials desired for start up, run and passage of a splice, contribute to other problems, such as excess coverage, bead pull-through, pencil lines, crosslines, breaklines and streaks, encountered in the manufacture of photographic film.
It is an object of the present invention to overcome the above problems.